In recent years, camera modules for taking photos have begun to be incorporated in portable terminals such as mobile phones and laptop computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be mass manufactured and marketed. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pickup devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modern image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, and excellent optical performance.
Compactness means in particular that a length from a lens edge of the lens system to an image pick-up surface should be as short as possible.
Low cost means in particular that the lens system should include as few lenses as possible; and that the lenses should be able to be formed from a resin or a plastic and be easily assembled.
Excellent optical performance can be classified into the following four main requirements:
First, a high brightness requirement, which means that the lens system should have a small F number (FNo.). Generally, the FNo. should be 2.8 or less.
Second, a wide angle requirement, which means that a half of a field of view of the lens system should be 30° or more.
Third, a uniform illumination over the image surface requirement, which means that the lens system can reduce or even avoid the occurrence of vignetting, thereby enhance illumination over edge portions of the image surface.
Fourth, a high resolution requirement, which means that the lens system should appropriately correct fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration.
In a single lens system which satisfies the low cost and lightweight requirement, the single lens of the system is generally made from a plastic. However, it is difficult for the single lens system to effectively correct chromatic aberration and achieve excellent optical performance, especially in the case where a wide angle of view of for example 70° is desired. Therefore, employment of a diffraction lens to correct chromatic aberration was proposed in an article entitled “Optical Design of Diffraction Lens System” (Yunguang Technology, Vol. 32, No. 2, 2000, pp. 33-39). In this article, a hybrid diffractive-refractive single lens system is employed to correct chromatic aberration. Other similar lens systems are disclosed in U.S. Pat. No. 6,055,105B1, U.S. Pat. Application Publication No. US2003/0117709A1, and EP Pat. No. 0819952A2. However, none of these lens systems can achieve excellent optical performance in the case of a wide angle of view. In addition, the distortion, field curvature and astigmatism of the lens systems cannot be effectively corrected.
Typical two-lens systems can be found in U.S. Pat. Application Publication Nos. 2003/0117723 and 2004/0036983, and EP Pat. No. 1357414A1. In order to correct chromatic aberration, two lenses are employed in the lens system. The two lenses must be made from different materials, with Abbe constants thereof differing significantly. The number of the glass/plastic lens(es) in the single lens system and the two-lens system is insufficient to correct the distortion, field curvature and astigmatism that typically occur in such lens systems. In other words, such lens systems generally cannot provide sufficiently excellent optical performance. The single lens system and two-lens system are generally used in a low-resolution image pickup device such as a CMOS.
In order to satisfy the excellent optical performance requirement, three lenses are generally employed. Three-lens systems can provide a wider angle of view and correct various aberrations. A three-lens system comprises, from an object side to an image side, an aperture stop, a first lens having positive focal power, a second lens having negative focal power, and a third lens having positive focal power. Typical three-lens systems can be found in U.S. Pat. Application Publication No. 2003/0193605A1, and Japanese Patent Publication No. 2001-075006. The lens system shortens the distance between the object side and the image side. The aperture stop is arranged between the object side and the first lens, for narrowing down an incident angle of chief rays being projected onto the image pick-up surface. In order to correct chromatic aberration, the lenses that have positive refracting power must be made from different materials, with the corresponding Abbe constants differing significantly.
In a lens system which satisfies the low cost requirement, all lenses of the three-lens system should be made from a plastic or a resin material. However, there are only a few varieties of plastic materials which can be suitably used to make lenses. Even if all the lenses are made from a different plastic or resin material, the range of variation of optical properties of the lenses is limited. This makes it difficult to effectively correct chromatic aberration. Another important consideration is that plastic materials are prone to absorb water. For example, the water absorbency of polymethyl methacrylate (PMMA) is 1.5%, and the water absorbency of polycarbonate (PC) is 0.4%. Among the plastic or resin materials which can be suitably used to make lenses, only zeonex materials (polyolefin resins or cyclo-olefin polymers) have relatively low water absorbency. In general, the water absorbency is less than 0.01%. Zeonex materials are available from the Japanese Zeon Corporation. Therefore unless a non-glass lens is made from a zeonex material, it is liable to absorb water and deform. As a result, the optical performance of the lens system is diminished.
Therefore, a low cost image pick-up lens system which has a compact size and excellent optical performance is desired.